202 Mr. A. M. Herbert; on the Effect of Hydrogen on 



place at different times of the day and under different atmo- 

 spheric conditions, and thus escaping the notice of even an 

 expert observer, Rayleigh's formula regarding the composition 

 of the diffuse light of the sky is not verified numerically, it 

 does not follow that Rayleigh's theory is not to be retained 

 as furnishing the best explanation of the colour of the 

 atmosphere. Variations in the number and size of the par- 

 ticles which give rise to the turbidity of the atmosphere, and 

 in what is still more irregular — equally invisible strata of 

 water-vapour in the lower regions — explain sufficiently, in 

 accordance with Rayleigh's theory, the discrepancies among 

 the complex results which have been obtained in the strata 

 just above the surface of the soil — strata forming ' atmo- 

 spheric mud.' 



"At the present time, the controversial choice lies between 

 the colour proper of the air and Rayleigh's theory. This 

 theory has an energetic and staunch supporter in Prof. Pernter, 

 who, as we have seen, has several times replied to Spring, 

 combating the latter's views. In the present state of science, 

 Rayleigh's theory is the only one capable of explaining satis- 

 factorily certain observed facts of special importance in con- 

 nexion with the colour of the atmosphere, of which the 

 principal one is that at an angle of 90° we get the maximum 

 of polarization by the atmosphere, and of light reflected 

 by a turbid medium — a fact which, according to Pernter, 

 4 is the experimentum crucis with regard to the theory of the 

 blue colour of the sky.' 



" 1 shall conclude by quoting Pernter : — 



" ' The turbid medium called air is that which gives rise to 

 the blue colour of the sky. The weak colour proper of the 

 air — if it exists — contributes nothing towards it/ " 



XX. The Effect of the Presence of Hydrogen on the Intensity 

 of the Lines of the Carbon Spectrum. By Arthur M. 

 Herbert, B.A* 



TN a paper "On the Spectra of some of the Chemical 

 A Elements," in the Philosophical Transactions for 1864, 

 Sir William Huggins remarks that " when carbon is sub- 

 jected to the induction-spark in the presence of hydrogen 

 the strong line in the red (a little less refrangible than the 

 hydrogen red line corresponding to Fraunhofer's C in the 

 solar spectrum) is not seen." This peculiarity is referred to 

 by him once again in connexion with certain cometary spectra 



* Communicated by Prof. Arthur Schuster, F.R.S. 



